Global $Λ$ hyperon polarization in nuclear collisions: evidence for the most vortical fluid (1701.06657v1)

Abstract: The extreme temperatures and energy densities generated by ultra-relativistic collisions between heavy nuclei produce a state of matter with surprising fluid properties. Non-central collisions have angular momentum on the order of 1000$\hbar$, and the resulting fluid may have a strong vortical structure that must be understood to properly describe the fluid. It is also of particular interest because the restoration of fundamental symmetries of quantum chromodynamics is expected to produce novel physical effects in the presence of strong vorticity. However, no experimental indications of fluid vorticity in heavy ion collisions have so far been found. Here we present the first measurement of an alignment between the angular momentum of a non-central collision and the spin of emitted particles, revealing that the fluid produced in heavy ion collisions is by far the most vortical system ever observed. We find that $\Lambda$ and $\overline{\Lambda}$ hyperons show a positive polarization of the order of a few percent, consistent with some hydrodynamic predictions. A previous measurement that reported a null result at higher collision energies is seen to be consistent with the trend of our new observations, though with larger statistical uncertainties. These data provide the first experimental access to the vortical structure of the "perfect fluid" created in a heavy ion collision. They should prove valuable in the development of hydrodynamic models that quantitatively connect observations to the theory of the Strong Force. Our results extend the recent discovery of hydrodynamic spin alignment to the subatomic realm.

• The paper demonstrates that Λ and anti-Λ hyperons show positive polarization (~1%), providing direct evidence of QGP vorticity.
• Experimental data from non-central gold nucleus collisions at RHIC indicate a record vorticity of about 9×10^21 s⁻¹.
• These findings refine hydrodynamic models and open new avenues in understanding quantum chromodynamics under extreme conditions.

Global Λ Hyperon Polarization in Nuclear Collisions: Observations and Implications

The paper "Global Λ Hyperon Polarization in Nuclear Collisions: Evidence for the Most Vortical Fluid" presents seminal findings from experiments at the Relativistic Heavy Ion Collider (RHIC) that provide concrete evidence for the vortical nature of the quark-gluon plasma (QGP) produced in non-central heavy ion collisions. This research marks a significant step toward understanding the properties of the QGP and its rotational dynamics, which are crucial for the paper of quantum chromodynamics (QCD) under extreme conditions.

The primary experimental evidence presented involves the measurement of the polarization of Λ and $\overline{\Lambda}$ hyperons in relation to the angular momentum of non-central collisions between gold nuclei. A positive polarization on the order of a few percent was observed, aligning with hydrodynamic predictions and pointing to the presence of strong vorticity in the created fluid. These findings challenge previous assumptions, as they offer the first experimental observation connecting the angular momentum of such nuclear collisions to the spin orientation of emitted particles, hence confirming the QGP's vortical nature.

Key Findings and Numerical Results

1. Hyperon Polarization:
   • Both Λ and $\overline{\Lambda}$ hyperons exhibited a positive polarization consistent with hydrodynamically predicted values.
   • The polarization levels for Λ and $\overline{\Lambda}$ hyperons were observed to be energy-independent and averaged at 1.08% and 1.38%, respectively.
2. Vorticity Estimations:
   • The deduced vorticity of the fluid, from the Λ and $\overline{\Lambda}$ polarization, is approximately $9 \times 10^{21} \, \text{s}^{-1}$. This value far surpasses that of any other known fluid, indicating the QGP as the most vortical fluid ever observed.

Implications and Future Directions

The implications of this research are multifaceted, both in practical and theoretical contexts. Practically, the measurement of hyperon polarization serves as a direct probe into the vortical structure of the QGP, which is critical for refining hydrodynamic models that describe the system created in such extreme conditions. The demonstration of such extreme vorticity in heavy ion collisions also opens avenues for exploring novel QCD effects associated with chiral symmetry restoration, which could offer insights into fundamental processes like quark confinement and the formation of hadronic mass.

On a theoretical level, understanding the vorticity in QGP contributes to comprehending the role of angular momentum in fluid dynamics at the subatomic level. This observation aligns with emerging theories that posit significant interactions between vorticity and magnetic fields, potentially leading to phenomena such as the chiral magnetic effect.

In conclusion, these findings lay the groundwork for future exploration into the QGP’s properties at RHIC and other high-energy colliders. With continued refinement of experimental techniques and enhanced statistical sampling, future studies may elucidate further the connection between macroscopic angular momentum and microscopic spin phenomena, enhancing our understanding of QCD under extreme conditions and contributing broadly to the field of nuclear physics.